1. Field of the Invention
The present invention relates to an exposure apparatus used when producing for example a semiconductor device, an imaging device, a liquid crystal display, a thin film magnetic head, or another microdevice and to a photomask and aberration correction plate used for the exposure apparatus and a process of production of such a microdevice.
2. Description of the Related Art
In the photolithographic process for producing a semiconductor device etc., use is made of an exposure apparatus to transfer a pattern image of a photomask (including reticle) on to a photosensitive substrate through a projection optical system. The photomask used for such an exposure apparatus is generally produced using silica glass (quartz glass). Silica glass has a low transmission loss of light, has resistant to temperature changes, is excellent in corrosion resistance and elastic performance, has a small coefficient of linear expansion (about 5.5xc3x9710xe2x88x927/K), and has other superior properties. It therefore has the advantages of enabling formation of a pattern with a high precision and a good work efficiency. Semiconductor integrated circuits of increasing miniaturization are being developed. In the photolithographic process, the wavelengths of the light sources used are becoming increasingly shorter. Vacuum ultraviolet light, in particular, light of wavelengths shorter than 200 nm, for example, light of ArF excimer lasers (wavelength 193 nm) or F2 lasers (wavelength 157 nm), etc. has now come into use as exposure light.
If ordinary silica glass is used for light of a wavelength less than about 200 nm, however, the transmission loss increases due to absorption and scattering. Further, the optical performance declines due to heat buildup resulting from absorption and to the fluorescence. The glass itself discolors along with time, that is, xe2x80x9ccolor centerxe2x80x9d occurs, and changes in density, that is, xe2x80x9ccompactionxe2x80x9d, occurs. These become more remarkable the shorter the wavelength. Therefore, when using ordinary silica glass, it has been thought that use for light of a wavelength about that of an ArF excimer laser (wavelength 193 nm) was the limit. It was generally considered difficult to use a photomask made of ordinary silica glass for light of shorter wavelengths. Therefore, production of a photomask using fluorite (CaF2), a material with a high transmittance even with respect to light of a wavelength less than about 200 nm, has been studied. Note that the use of fluorite is considered particularly effective when used for light of a wavelength less than 190 nm, but use of fluorite is also preferable in the case of the above ArF excimer laser from the viewpoint of the transmittance.
The Mohs hardness of fluorite, however, is 4. This is softer than the Mohs hardness 7 of quartz, the material used for photomask substrates in the past. Use as a photomask is problematic in that the photomask is susceptible to damage during transport and the dust generated from the damaged locations is liable to cause unwanted foreign matter to deposit on the pattern and prevent exposure of the desired pattern. Therefore, when producing a photomask using fluorite, measures have to be taken against this. Further, fluorite has a low resistance to temperature changes, is fragile and easily scratched, is damaged during the process of pattern formation, and has a higher coefficient of linear expansion compared with quartz, so special control and countermeasures are required for forming a mask pattern with a high precision. Further, when used for an exposure apparatus for actual exposure work, the temperature has to be controlled extremely strictly. Sometimes obstacles arise in transferring a pattern on a photosensitive substrate with a higher precision.
The above problems relate to the photomask per se. The exposure apparatus is sometimes equipped with a fixed or detachable transparent aberration correction plate in the path of the imaging light between the photomask and photosensitive substrate (for example, between the photomask and projection optical system) so as to correct distortion resulting from distortion of the projection optical system or other aberration. Similar problems arise as with the above photomask for this aberration correction plate as well. Further, the problems relating to the photomask and aberration correction plate hinder shortening of the wavelength of the exposure apparatus, make dealing with the increasing miniaturization of patterns to be formed on photosensitive substrates difficult, and make realization of stable exposure performance over time and provision of an exposure apparatus with a long service life hard.
Accordingly, a first object of the present invention is to provide a photomask produced using a material, such as fluorite, having a low hardness to enable the exposure wavelength to be shortened further and a finer pattern to be transferred, yet which is not damaged when transporting the photomask or at the time of scan exposure.
A second object of the present invention is to provide a photomask, used for an exposure apparatus using light of a particularly short wavelength (wavelength of not more than about 200 nm) for exposure, which is high in light transmittance, excellent in UV resistance, good in processability, and enables realization of a high precision.
A third object of the present invention is to provide an aberration correction plate, used for an exposure apparatus using light of a particularly short wavelength (wavelength of not more than about 200 nm) for exposure, which is high in light transmittance, excellent in UV resistance, good in processability, and enables realization of a high precision.
A fourth object of the present invention is to provide an exposure apparatus able to handle the increasing miniaturization of patterns formed on photosensitive substrates, feature low deterioration of the exposure performance along with time, and having a long service life.
A fifth object of the present invention is to provide a process for production of a microdevice of a good quality.
1. To achieve the first object, according to the present invention, there is provided a photomask including a substrate, a transfer pattern formed on the substrate, and a protective film formed on the substrate for protecting the substrate. The protective film may be formed, in a region of the outer surface of the substrate other than the region at which the transfer pattern is formed, at a contact surface with a member for holding the photomask and at least at a part of the area near the contact surface.
According to the photomask of the present invention, since a protective film is formed for protecting the substrate of the photomask, even if the photomask is produced using a material, such as fluorite, with a low hardness, the photomask will not be damaged during transport of the photomask or during scan exposure. The photomask is preferably irradiated with illumination light of a wavelength not more than 190 nm. In this case, it is possible to transfer an extremely finer pattern using extreme ultraviolet illumination light of a wavelength not more than 190 nm. The substrate of the photomask is preferably formed by calcium fluoride (CaF2). Note that fluorite is a typical example of this. In this case, it is possible to use extreme ultraviolet exposure light such as light of an F2 excimer laser (wavelength 157 nm). The protective film is preferably comprised of chrome (Cr), chromium oxide (CrO), or silicon oxide (SiO2 or SiO). In this case, it is possible to keep down the cost for forming the protective film for preventing damage of the photomask.
The protective film may be formed by the same material as the pattern. Further, the protective film may be provided at the surface of the substrate on which the pattern is formed or on an end face of the substrate.
2. To achieve the second object, according to the present invention, there is provided a photomask, used for an exposure apparatus having a light source emitting illumination light of a wavelength not more than about 200 nm, including a substrate comprised of fluorine-doped silica glass and a pattern formed on that substrate. The silica glass may be produced by hydrolyzing a silicon compound in a flame, causing the glass particles obtained to deposit to form porous glass, heat treating the porous glass in a fluorine-containing atmosphere to dope it with fluorine, then making it transparent.
Fluorine-doped silica glass has incomplete structures (bonds) in the glass terminated by fluorine, so becomes more complete in structure. There is therefore less absorption of ultraviolet light and the structure is stably maintained even if exposed to the strong energy of ultraviolet rays. Therefore, a photomask high in light transmittance and with little deterioration along with time due to exposure to ultraviolet rays (color center, compaction, etc.) when using ultraviolet rays of a wavelength not more than about 200 nm as the illumination light is provided. Further, compared with a photomask produced using fluorite, there is resistance to heating and cooling and resistance to scratching, so the work efficiency at the time of production, including pattern formation, is good and the coefficient of linear expansion is extremely small. Consequently, a high precision photomask is provided.
Further, the silica glass is preferably further doped with hydrogen. This is because by doping with hydrogen in addition to fluorine, bonds which are cleaved by exposure to ultraviolet rays are terminated by the doped hydrogen atoms, so the UV resistance can be further improved and, by doing this, a photomask with little deterioration due to exposure to ultraviolet rays is provided. Further, the concentration of OH groups in the silica glass is preferably not more than 100 ppm. This is because when using the photomask of the present invention in an exposure apparatus provided with a light source emitting illumination light of a wavelength not more than about 170 nm, keeping down the concentration of OH groups in the silica glass to less than 100 ppm enables an improvement of the transmittance with respect to such short wavelength light.
3. To achieve the second object, according to the present invention, there is provided a photomask, used for an exposure apparatus provided with a light source emitting illumination light of a wavelength not more than about 200 nm, including a substrate produced using silica glass having a structure determining temperature of not more than 1200 K and an OH group concentration of not less than 1000 ppm and a pattern formed on the substrate.
Silica glass containing not less than 1000 ppm of OH groups is structurally stable compared with other silica glass and tends to have a lower structure determining temperature. Further, a low structure determining temperature is believed to mean less distorted bonded portions in the glass, so there is less cleavage of distorted bonded portions due to exposure to ultraviolet rays and therefore less of a scattering loss or absorption loss and an improvement of the UV resistance. Accordingly, there is provided a photomask with a high light transmittance and low deterioration along with time due to exposure to ultraviolet rays (color center, compaction, etc.) when using ultraviolet rays of a wavelength not more than about 200 nm as the exposure light. Further, compared with a photomask produced using fluorite, there is resistance to heating and cooling and resistance to scratching, so the work efficiency in production, including pattern formation, is excellent and the coefficient of linear expansion is extremely small. Therefore, a high precision photomask is provided.
Note that the xe2x80x9cstructure determining temperaturexe2x80x9d spoken of here is a factor introduced as a parameter expressing the structural stability of silica glass. The fluctuation in density of silica glass at room temperature, that is, the structural stability, is determined by the density and structure when the silica glass in the molten state at a high temperature freeze near the glass transition point in the process of cooling. That is, the thermodynamic density and structure corresponding to the temperature at which the density and structure freeze are preserved even under room temperature. The temperature at which the density and structure freeze is defined as the xe2x80x9cstructure determining temperaturexe2x80x9d.
4. To achieve the second object, according to the present invention, there is provided a photomask, used for an exposure apparatus having a light source emitting illumination light of a wavelength not more than about 200 nm, including a substrate comprised of a thin sheet of silica glass, a pattern formed on the substrate, and a reinforcing member, for holding the substrate so as to prevent deformation of the substrate, comprised of a material having a transmittance at least equal to the substrate at least at the area where the illumination light passes. The silica glass is preferably fixed so that the surface on which the pattern is formed is in substantially close contact with the reinforcing member. As the material of the reinforcing member, any of fluorine-doped silica glass, silica glass with an OH group concentration of not less than 1000 ppm, calcium fluoride, rock crystal, and magnesium fluoride may be used.
The loss of light passing through the silica glass becomes larger in proportion to the thickness of the glass, so the glass is made a thin sheet and the pattern formed on there so as to reduce the transmission loss at that portion. Further, by holding the thin sheet of silica glass by a reinforcing member comprised of a material having a transmittance at least equal to that of the silica glass at least at the area where the illumination light passes, the low strength resulting from the thin sheet shape is compensated for and flexing etc. are prevented. Since the pattern is formed on the thin sheet of silica glass, there is little loss even when using ultraviolet rays of a wavelength not more than about 200 nm as the illumination light. Further, there is less deterioration due to heating and cooling and the coefficient of linear expansion is small as well, so a high precision pattern can be easily formed. Further, the silica glass on which the pattern is formed is reinforced by a reinforcing member, so there is little flexing and a photomask high in precision and excellent in characteristics is provided. The silica glass is preferably doped with fluorine. This is because by doping the thin sheet of silica glass with fluorine, the transmission loss can be further reduced and the UV resistance can be improved.
5. To achieve the fourth object, according to the present invention, there is provided an exposure apparatus comprising an illumination optical system for emitting illumination light of a wavelength of less than about 200 nm to a photomask, a projection optical system for projecting the illumination light emitted from the photomask on to a photosensitive substrate, a stage arranged at the object plane side of the projection optical system and moving the photomask substantially along the object plane, and a reinforcing member, provided at the stage for holding the photomask and preventing its deformation, comprised of a material having a transmittance at least equal to that of the photomask at least at an area through which the illumination light passes. The photomask is preferably fixed so that the surface on which the pattern is formed is substantially in close contact with the reinforcing member. As the material of the reinforcing member, any of fluorine-doped silica glass, silica glass with an OH group concentration of not less than 1000 ppm, calcium fluoride, rock crystal, and magnesium fluoride may be used.
According to this exposure apparatus, since the photomask is held by a reinforcing member comprised of a material having a transmittance at least equal to it, flexing or other deformation of the photomask are prevented and therefore a high precision of pattern transfer can be realized. Further, since flexing and other deformation are prevented even if the photomask is made thin, it is possible to reduce the transmission loss without inviting a drop in precision.
6. To achieve the third object, according to the present invention, there is provided an aberration correction plate of a projection optical system, projecting an image of a pattern to be exposed to illumination light of a wavelength not more than about 200 nm on to a predetermined surface, produced using fluorine-doped silica glass or silica glass having a structure determining temperature of not more than 1200 K and an OH group concentration of not less than 1000 ppm.
Fluorine-doped silica glass has incomplete structures (bonds) in the glass terminated by fluorine, so becomes more complete in structure. There is therefore less absorption of ultraviolet light and the structure is stably maintained even if exposed to the strong energy of ultraviolet rays. Further, silica glass containing not less than 1000 ppm of OH groups is structurally stable compared with other silica glass and tends to have a lower structure determining temperature. Further, a lower structure determining temperature is believed to mean less distorted bond portions in the glass, therefore there is less cleavage of distorted bond portions due to exposure to ultraviolet rays and therefore there is less scattering loss or absorption loss and the UV resistance is improved. Therefore, an aberration correction plate high in light transmittance and with little deterioration along with time due to exposure to ultraviolet rays (color center, compaction, etc.) when using ultraviolet rays of a wavelength not more than about 200 nm as the illumination light is provided. Further, since it is scratch resistant, the work efficiency at the time of production, including the grinding to a shape to deal with the distribution of distortion, is good and the coefficient of linear expansion is extremely small, so an aberration correction plate which can correct aberration with a high precision is provided.
7. To achieve the fourth object, according to the present invention, there is provided an exposure apparatus comprising an illumination optical system for emitting illumination light of a wavelength not more than about 200 nm to a photomask, a projection optical system for projecting the illumination light emitted from the photomask on to a photosensitive substrate, and an aberration correction plate, comprised of fluorine-doped silica glass, for correcting nonrotationally symmetric aberration of the projection optical system and arranged in the path of imaging light between the photomask and photosensitive substrate.
According to this exposure apparatus, since provision is made of an aberration correction plate using a predetermined silica glass and this aberration correction plate has the superior characteristics of a high light transmittance with respect to irradiation by ultraviolet rays of a wavelength not more than about 200 nm and little deterioration along with time, it is possible to correct aberration stably over time. Therefore, it is possible to realize a high exposure precision over a long period, improve the reliability of the apparatus, and increase the service life.
8. To achieve the second object, according to the present invention, there is provided a photomask comprising a substrate comprised of a sheet of rock crystal and a pattern formed on the substrate.
Rock crystal, that is, crystallized quartz, has a low absorption even in the vacuum ultraviolet spectrum (same for infrared spectrum and visible light spectrum) and can maintain its structure even if exposed to the strong energy of ultraviolet rays. Therefore, a photomask high in light transmittance and with little deterioration along with time due to exposure to ultraviolet rays even when using ultraviolet rays of an extremely short wavelength (for example, a wavelength not more than about 200 nm) as the illumination light is provided. Further, compared with a photomask produced using fluorite, there is resistance to heating and cooling and resistance to scratching, so the work efficiency at the time of production, including pattern formation, is good and the coefficient of linear expansion is small. Consequently, a high precision photomask is provided.
9. To realize the fourth object, according to the present invention, there is provided an exposure apparatus comprising a photomask formed with a pattern to be transferred to a rock crystal sheet, an illumination optical system for emitting illumination light to the photomask, and a projection optical system for projecting the illumination light emitted from the photomask on to a photosensitive substrate.
The photomask used for the exposure apparatus of the present invention is comprised of rock crystal, that is, crystallized quartz. Rock crystal has a low absorption even in the vacuum ultraviolet spectrum and can maintain its structure even if exposed to the strong energy of ultraviolet rays. Further, as the light source, it is possible to use one emitting ultraviolet rays of an extremely short wavelength (for example, a wavelength not more than about 200 nm), so it is possible to deal with the increasing miniaturization of patterns formed on photosensitive substrates, stably maintain the exposure performance over a long period, and increase the service life.
10. To achieve the fifth object, according to the present invention, there is provided a process of production of a microdevice comprising bringing a protective film formed on a mask together with a circuit pattern into contact with a projection to hold the mask, emitting illumination light of a wavelength not more than about 200 nm to the held mask, using the illumination light from through the mask for exposing a photosensitive layer on the substrate on which the microdevice is to be formed, and thereby transferring the circuit pattern on to the substrate.
According to this process of production of a microdevice of the present invention, since the pattern is transferred while holding the mask by bringing a projection into contact with a portion on which the protective film is formed, damage and generation of dust along with holding the mask are prevented, deposition of broken pieces etc. on the mask or other optical elements etc. and formation of a poor pattern are prevented, and a microdevice having good quality can be produced.
11. To achieve the fifth object, according to the present invention, there is provided a process of production of a microdevice comprising emitting illumination light of a wavelength not more than about 200 nm on a mask made using any of fluorine-doped silica glass, silica glass with an OH group concentration of not less than 1000 ppm, magnesium fluoride, and rock crystal, using the illumination light from the mask for exposing a photosensitive layer on a substrate on which the microdevice is to be formed, and thereby transferring the pattern of the mask on the substrate. In this case, when illuminating the photosensitive layer through a projection optical system and using a mask comprised of rock crystal, the mask should be arranged so that its optical axis becomes substantially parallel with the optical axis of the projection optical system.
According to the process of production of a microdevice of the present invention, there is little deterioration of the mask over time even if using illumination light of a short wavelength, so it is possible to produce high quality microdevices stably over a long period.